Change IP. Stay connected.

Mosh automatically roams as you move between Internet
connections. Use Wi-Fi on the train, Ethernet in a hotel,
and LTE on a beach: you'll stay logged in. Most network
programs lose their connections after roaming,
including SSH and Web apps like Gmail. Mosh
is different.

Makes for sweet dreams.

With Mosh, you can put your laptop to sleep and wake it
up later, keeping your connection intact. If your
Internet connection drops, Mosh will warn you — but
the connection resumes when network service
comes back.

Get rid of network lag.

SSH waits for the server's reply before showing you your
own typing. That can make for a lousy user interface. Mosh
is different: it gives an instant response to typing,
deleting, and line editing. It does this adaptively and
works even in full-screen programs like emacs and vim. On
a bad connection, outstanding predictions are underlined
so you won't be misled.

No privileged code. No daemon.

You don't need to be the superuser to install or run
Mosh. The client and server are executables run by an
ordinary user and last only for the life of the
connection.

Same login method.

Mosh doesn't listen on network ports or authenticate
users. The mosh client logs in to the server via
SSH, and users present the same credentials (e.g.,
password, public key) as before. Then Mosh runs the
mosh-server remotely and connects to it over UDP.

Runs inside your terminal, but better.

Mosh is a command-line program, like ssh. You can use it
inside xterm, gnome-terminal, urxvt, Terminal.app, iTerm,
emacs, screen, or tmux. But mosh was designed from scratch
and supports just one character set: UTF-8. It fixes Unicode
bugs in other terminals and in SSH.

Recent News

July 21, 2017: Mosh 1.3.2 released, with John Hood as
release lead. The release includes improved tests, bug fixes,
and improvements to IPv6 support on non-Linux systems.

March 25,
2017: Mosh
1.3.0 released, with John Hood as release lead. The release
includes broader platform compatibility, robustness improvements,
better testing, and fixes for excess CPU consumption in some cases. We
have switched to semver.org-style versioning and will increment the
minor version number whenever we add new functionality. (In our
previous practice, this release would probably have been called
“1.2.7.”)

August 10,
2016: Mosh
1.2.6 released, with John Hood as release lead. New features
include huge performance improvements, especially on large terminals,
the ability to set a timeout to end dormant sessions automatically,
and support for crypto libraries other than OpenSSL.

January 20, 2014: Mosh for Chrome, which brings Mosh to the Chrome browser and Chrome OS, is released. It can be installed here.

August 9, 2013: JuiceSSH (SSH client for Android) adds official Mosh support — available on the Play Store

April 14, 2013: Mosh has posted an Ideas List for interested contributors!

March 27,
2013: Mosh
1.2.4 has been released. Changes largely include bug
fixes, improved robustness, and added platform support (now
on AIX and stock Solaris!). This version will be in Ubuntu
13.04 (raring).

Oct. 19,
2012: Mosh
1.2.3 has been released. Changes include more resilience to
evil NATs, power savings for mobile clients, switching to OpenSSL's AES
implementation, and a licensing exception to allow Mosh on Apple's app store.
This version will be in Debian 7.0 (wheezy).

Install from your package manager

macOS 10.10 or later

This is a standalone OS X package that will work on any supported Macintosh. However, if you are using a package manager such as Homebrew or MacPorts, we suggest using it to get Mosh, for better compatibility and automatic updates.

Homebrew OS X 10.5 or later

MacPorts OS X 10.5 or later

Windows

There is no "native" mosh executable for Windows available at this time. The Chrome version of Mosh is the easiest way to use mosh on Windows.

Cygwin

C:\> setup.exe -q mobile-shell

Mosh on Cygwin uses OpenSSH and is suitable for Windows users with advanced SSH configurations.
Mosh is not compatible with Cygwin's built-in Windows Console terminal emulation. You will need to run Mosh from a full-featured terminal program such as mintty, rxvt, PuTTY, or an X11 terminal emulator.

Security on new operating systems

Note that mosh-client receives an AES session key as an environment
variable. If you are porting Mosh to a new operating system, please make sure that a
running process's environment variables are not readable by other users. We have
confirmed that this is the case on GNU/Linux, OS X, and FreeBSD.

Usage

Typical usage

Mosh will log the user in via SSH, then start a connection on a UDP port between 60000 and 61000.

Different username

$ mosh potus@ackbar.bls.gov

Server binary outside path

$ mosh --server=/tmp/mosh-serverr2d2

The user can specify an alternate path for the mosh-server on the remote host. The server binary can even
be installed in the user's home directory.

Selecting Mosh UDP port

$ mosh -p 1234darth

Useful when the server is behind a port-forwarder or NAT.

Selecting SSH port

$ mosh --ssh="ssh -p 2222"figrindan

Other SSH options

$ mosh --ssh="~/bin/ssh -i ./identity"fett

Disable instant echo

$ mosh --predict=neverniennunb

The -n switch is a synonym. By contrast,
passing --predict=always or -a
will enable instant local echo even on low-delay
links.

With a command

$ mosh pello-- screen -dr

This reattaches to a long-running screen session.

Ending the connection

Normally, logout or exit on the remote host will close
the session. Mosh accepts the escape sequence Ctrl-^
. (typically typed with Control-Shift-6, then a
period) to end the connection forcibly. To send a
literal Ctrl-^, type Ctrl-^ ^.

Manual

More details can be found in
the mosh(1), mosh-client(1),
and mosh-server(1) manual pages.

(Why you should trust Mosh with your remote terminal needs: we
worry about details so obscure, even USENIX reviewers don't want to
hear about them.)

How Mosh works

Remote-shell protocols traditionally work by conveying a
byte-stream from the server to the client, to be interpreted
by the client's terminal. (This includes TELNET, RLOGIN, and
SSH.) Mosh works differently and at a different layer. With
Mosh, the server and client both maintain a snapshot of
the current screen state. The problem becomes one of
state-synchronization: getting the client to the
most recent server-side screen as efficiently as
possible.

This is accomplished using a new protocol called the
State Synchronization Protocol, for which Mosh is the
first application. SSP runs over UDP, synchronizing the
state of any object from one host to another. Datagrams
are encrypted and authenticated
using AES-128
in OCB3 mode. While SSP takes care of the networking
protocol, it is the implementation of the object being
synchronized that defines the ultimate semantics of the
protocol.

Roaming with SSP becomes easy: the client sends datagrams
to the server with increasing sequence numbers, including
a "heartbeat" at least once every three seconds. Every time
the server receives an authentic packet from the client
with a sequence number higher than any it has previously
received, the IP source address of that packet becomes the
server's new target for its outgoing packets. By doing
roaming “statelessly” in this manner, roaming works in and
out of NATs, even ones that may themselves be
roaming. Roaming works even when the client is not aware
that its Internet-visible IP address has changed. The
heartbeats allow Mosh to inform the user when it hasn't
heard from the server in a while (unlike SSH, where users
may be unaware of a dropped connection until they try to
type).

Mosh runs two copies of SSP, one in each direction of the
connection. The connection from client to server
synchronizes an object that represents the keys typed by
the user, and with TCP-like semantics. The connection from
server to client synchronizes an object that represent the
current screen state, and the goal is always to convey the
client to the most recent server-side state, possibly
skipping intermediate frames.

Because SSP works at the object layer and can control the
rate of synchronization (in other words, the frame rate),
it does not need to send every byte it receives from the
application. That means Mosh can regulate the frames so as
not to fill up network buffers, retaining the
responsiveness of the connection and making sure Control-C
always works quickly. Protocols that must send every byte
can't do this.

Careful terminal emulation

One benefit of working at the terminal layer
was the opportunity to build a clean UTF-8 terminal
emulator from scratch. Mosh fixes several Unicode bugs in
existing terminals and in SSH, and was designed as a fresh
start to try to be robust and correct even for
pathological inputs.

Tricky unicode

Only Mosh and the OS X Terminal correctly handle a Unicode combining character in the first column.

xterm: circumflex on wrong letter.

GNOME Terminal: no circumflex at all.

OS X Terminal.app gets it right.

Mosh gets it right too.

ISO 2022 locking escapes

Only Mosh will never get stuck in hieroglyphs when a nasty program writes to the terminal. (See Markus Kuhn's discussion of the relationship between
ISO 2022 and UTF-8.)

xterm

GNOME Terminal

OS X Terminal.app

Mosh

Evil escape sequences

Only Mosh and GNOME Terminal have a defensible rendering when
Unicode mixes with an ECMA-48/ANSI escape sequence. The OS X Terminal
unwisely tries to normalize its input before the vt500 state machine,
causing it to misinterpret and become unusable after receiving the
following input!* (This also means the OS X Terminal's interpretation
of the incoming octet stream varies depending on how
the incoming octets are split across TCP segments, because the
normalization only looks ahead to available bytes.)

* We earlier wrote that this misbehaving sequence "crashes"
the OS X Terminal.app. This was mistaken—instead, Terminal.app
interprets the escape sequence as shutting off keyboard input, and
because of an unrelated bug in Terminal.app, it is not possible for
the user to restore keyboard input by resetting the terminal from the
menu.

xterm: circumflex on wrong letter.

GNOME Terminal's circumflex placement is defensible.

OS X Terminal.app applies circumflex to part of escape sequence, then irretrievably shuts off keyboard input.

Mosh gets this one right.

Mosh sets IUTF8

In the POSIX framework, the kernel needs to know whether
the user is typing in an 8-bit character set or in UTF-8,
because in canonical mode (i.e. "cooked" mode), the kernel
needs to be able to delete a typed multibyte character
sequence from an input buffer. On OS X and Linux, this is
done with the "IUTF8" termios flag.)
(See diagnostic
explaining the need for this flag.)

Mosh sets the IUTF8 flag when possible and stubbornly refuses to start up unless the user has a
UTF-8-clean environment. SSH does not set the IUTF8 flag, which can lead to garbage in input buffers.

Instant local echo and line editing

The other major benefit of working at the
terminal-emulation layer is that the Mosh client is free
to scribble on the local screen without lasting
consequence. We use this to implement intelligent local
echo. The client runs a predictive model in the background
of the server's behavior, hypothesizing that each
keystroke will be echoed at the cursor location and that
the backspace and left- and right-arrow keys will have
their traditional effect. But only when a prediction is
confirmed by the server are these effects actually shown
to the user. (In addition, by default predictions are only
displayed on high-delay connections or during a network
“glitch.”) Predictions are done in epochs: when the
user does something that might alter the echo behavior
— like hit ESC or carriage return or an up- or
down-arrow — Mosh goes back into making background
predictions until a prediction from the new batch can be
confirmed as correct.

Thus, unlike previous attempts at local echo with TELNET
and RLOGIN, Mosh's local echo can be used everywhere, even
in full-screen programs like emacs and vi.

Real-world benefits

We evaluated Mosh using traces contributed by six users, covering
about 40 hours of real-world usage and including 9,986 total
keystrokes. These traces included the timing and contents of all
writes from the user to the host and vice versa. The users were asked
to contribute "typical, real-world sessions." In practice, the traces
include use of popular programs such as the bash shell and zsh shells,
the alpine and mutt e-mail clients, the emacs and vim text editors,
the irssi and barnowl chat clients, the links text-mode Web browser,
and several programs unique to each user.

To evaluate typical usage of a "mobile" terminal, we replayed the
traces over an otherwise unloaded Sprint commercial EV-DO (3G)
cellular Internet connection in Cambridge, Mass. A client-side process
played the user portion of the traces, and a server-side process
waited for the expected user input and then replied (in time) with the
prerecorded server output. We speeded up long periods with no
activity. The average round-trip time on the link was about half a
second.

We replayed the traces over two different transports, SSH and Mosh,
and recorded the user interface response latency to each simulated
user keystroke. The Mosh predictive algorithm was frozen prior to
collecting the traces and was not adjusted in response to their
contents or results.

The results

Cumulative distribution of keystroke response times with Sprint EV-DO (3G) Internet service

Mosh reduced the median keystroke response
time from 503 ms to nearly instant (because more than 70% of the
keystrokes could be immediately displayed), and reduced the mean
keystroke response time from 515 ms to 173 ms. Qualitatively, Mosh makes
remote servers "feel" more like the local machine!

Frequently Asked Questions

Q: Who wrote Mosh?

Mosh was written by Keith Winstein, along with Anders Kaseorg, Quentin Smith, Richard Tibbetts, Keegan McAllister, and John Hood.

Q: Why another remote-terminal protocol?

Practical latency on the Internet is on the increase, with
the rise of bufferbloat and sophisticated wireless links
that optimize for throughput over delay. And roaming is more
common than ever, now that laptops and handheld devices have
largely displaced desktops. SSH is great, but frustrating to
use when you want to change IP addresses or have a
long-delay link or a dodgy connection.

Moreover, TELNET had some good things going for it — a
local-echo mode and a well-defined network virtual
terminal. Even today, SSH doesn't properly support UTF-8
end-to-end on a POSIX system.

Q: Are the mosh principles relevant to other network applications?

We think so. The design principles that Mosh stands for are
conservative: warning the user if the state being displayed
is out of date, serializing and checkpointing all
transactions so that if there are no warnings, the user
knows every prior transaction has succeeded, and handling expected events (like roaming from one
WiFi network to another) gracefully.

Those don't seem too controversial, but fancy apps like
Gmail-in-Chromium or on Android still behave atrociously on
dodgy connections or after switching IP addresses. (Have you
ever had Gmail leave an e-mail message in "Sending..." for ten
hours while merrily retrieving new mail and not indicating any
kind of error? Us too.) We think there may be considerable
room for improvement in many network user interfaces from the
application of these values.

Q: I'm getting "mosh requires a UTF-8 locale." How can I fix this?

To diagnose the problem, run locale on the local
terminal, and ssh remotehost locale. To use Mosh,
both sides of the connection will need to show a UTF-8 locale, like
LC_CTYPE="en_US.UTF-8".

On many systems, SSH will transfer the locale-related
environment variables, which are then inherited by
mosh-server. If this mechanism fails, Mosh (as of
version 1.2) will pass the variables itself. If neither
mechanism is successful, you can do something like

mosh remotehost--server="LANG=en_US.UTF-8 mosh-server"

If en_US.UTF-8 does not exist on the remote server,
you can replace this with a UTF-8 locale that does exist. You
may also need to set LANG locally for the benefit of
mosh-client. It is possible that the local and
remote machines will need different locale names. See also this GitHub
ticket.

Q: What does the message "Nothing received from the server on UDP port 60003" mean?

This means that mosh was able to start
mosh-server successfully on the remote machine, but the client is
not able to communicate with the server. This generally means that
some type of firewall is
blocking the UDP packets between the client and the server. If you
had to forward TCP port 22 on a NAT for SSH, then you will have to
forward UDP ports as well. Mosh will use the first available
UDP port, starting at 60001 and stopping at 60999. If you are only
going to have a small handful of concurrent sessions on a server, then you can
forward a smaller range of ports (e.g., 60000 to 60010).

Tools like netstat, netcat, socat, and tcpdump can be useful for debugging
networking and firewall problems.

This problem can also be the result of a bug in glibc 2.22 that
affects programs that link with protobuf and utempter and use
aggressive compiler hardening flags. (glibc bugtracker entry, as well as Mosh bugtracker entry.) The
problem causes mosh-server to segfault immediately on startup. We
believe we have worked around this problem in Mosh 1.2.6, but please
report a bug if you find otherwise.

Q: Why do you insist on UTF-8 everywhere?

We're really not UTF-8 zealots. But it's a lot easier to
correctly implement one terminal emulator
than to try to do the right thing in a variety of difficult
edge cases. (This is what GNU screen tries to do, and in our
experience it leads to some very tricky-to-debug situations.)
So mosh just won't start up until the user has everything
configured for a UTF-8-clean pathway. It may be annoying, but
it also probably reduces frustration down the
road. (Unfortunately an 8-bit vt220 and a UTF-8 vt220 are
different and incompatible terminal types; the UTF-8 goes
in underneath the vt220 state machine.)

Q: How do I use a different SSH port (not 22)?

As of Mosh 1.2, you can pass arguments to ssh like so:

mosh remotehost--ssh="ssh -p 2222"

Or configure a host alias in ~/.ssh/config with a
Port directive. Mosh will respect that too.

Q: I'm getting 'mosh-server not found'.

Please make sure that mosh is installed on the client, and
mosh (or at least mosh-server) is installed on the server you are
trying to connect to. Also, the server is expected to be available
on your server's default login PATH, which is not
usually true on OS X and BSD servers, or if you install mosh-server
in your home directory. In these cases please see the "Server
binary outside path" instructions in the Usage section,
above.

Q: SSH authenticates using Kerberos tickets, but Mosh asks me for a password.

In some configurations, SSH canonicalizes the hostname
before passing it to the Kerberos GSSAPI plugin. This breaks
for Mosh, because the initial forward DNS lookup is done by
the Mosh wrapper script. To work around this, invoke Mosh as

mosh remotehost--ssh="ssh -o GSSAPITrustDns=no"

This will
often fail on a round-robin DNS setup. In that case it is probably
best to pick a specific host from the round-robin pool.

Q: Why is my terminal's scrollback buffer incomplete?

Mosh synchronizes only the visible state of the terminal. We
are tracking this issue; see this issue and the
others which are linked from there. For now, the workaround is to use
screen or tmux on the remote side.

Q: How do I get 256 colors?

Make sure you are running mosh in a terminal that
advertises itself as 256-color capable. (This generally means
TERM will be xterm-256color or screen-256color-bce.)

Q: How do I type C-^, Mosh's default escape character?

On keyboards with the United States layout, this can be typed
as Ctrl-Shift-6, or often as Ctrl-6 (this depends on your OS and
terminal emulator). On non-US keyboards, it is often hard to find
the right key, and sometimes it's not available at all. If your
keyboard has a dead key with an accent-circumflex, this is not
likely to be the right key. Ctrl-6 sometimes works, though. If you
are unable to type this character, you will need to set
the MOSH_ESCAPE_KEY variable; see the Mosh man page for
details.

Q: How can I make the server automatically clean up dormant sessions?

Please see the entries for MOSH_SERVER_NETWORK_TMOUT
and MOSH_SERVER_SIGNAL_TMOUT in the mosh-server(1) man page.

Q: What is Mosh's security track record so far?

Mosh 1.0 was released in March 2012. As of the release of Mosh
1.3.2 in July 2017, as far as the developers are aware:

In the last four years, no security vulnerabilities of
any kind (major or minor) have been reported in Mosh.

No major security vulnerabilities have ever been
reported in Mosh. We define major security vulnerabilities to
include privilege escalation, remote code execution,
denial-of-service by a third party, etc.

Two denial-of-service issues were discovered and fixed in
releases in 2012. One issue allowed a mosh-server to cause the
mosh-client to spend excess CPU (CVE-2012-2385, fixed in Mosh
1.2.1, released May 2012). Another issue allowed the server
host to cause the mosh-client to send UDP datagrams to an
incorrect address, foiling its attempt to connect (fixed in
Mosh 1.2.3, released October 2012).

Q: How does Mosh's security compare with SSH's?

We think that Mosh's conservative design means that its attack
surface compares favorably with more-complicated systems like
OpenSSL and OpenSSH. Mosh's track record has so
far borne this out. Ultimately, however, only time will tell
when the first serious security vulnerability is discovered in
Mosh—either because it was there all along or because it
was added inadvertently in development. OpenSSH and OpenSSL have
had more vulnerabilities, but they have also been released
longer and are more prevalent.

In one concrete respect, the Mosh protocol is more secure than
SSH's: SSH relies on unauthenticated TCP to carry the contents
of the secure stream. That means that an attacker can end an SSH
connection with a single phony "RST" segment. By contrast, Mosh
applies its security at a different layer (authenticating every
datagram), so an attacker cannot end a Mosh session unless the
attacker can continuously prevent packets from reaching
the other side. A transient attacker can cause only a transient
user-visible outage; once the attacker goes away, Mosh will
resume the session.

However, in typical usage, Mosh relies on SSH to exchange keys
at the beginning of a session, so Mosh will inherit the
weaknesses of SSH—at least insofar as they affect the
brief SSH session that is used to set up a long-running Mosh
session.

Not that we know of—Mosh uses OCB3. The authors of the
paper write that the attack is not applicable to OCB3.

Q: Does mosh work with Amazon EC2?

Yes, it works great, but please remember to open up UDP ports 60000–61000 on the EC2 firewall.

Q: How do I tell if mosh is working correctly?

After you run mosh user@server, if successful you will be dropped into your login
shell on the remote machine.
If you want
to check that mosh is being used instead of ssh, try typing Ctrl-^ Ctrl-Z
to suspend the session (with mosh 1.2.4 or later on the client). Running fg will then return.

Q: What's the difference between mosh, mosh-client, and mosh-server? What one do I use?

The mosh command is a wrapper script that is designed to be the primary way that
you use mosh. In most cases, you can simply just replace "ssh" with "mosh" in your command line.
Behind the scenes, the mosh wrapper script will SSH to the server, start up
mosh-server, and then close the SSH connection. Then it will start up the
mosh-client executable on the client, passing it the necessary information for
it to connect to the newly spawned mosh-server instance.

In normal usage, mosh-client and
mosh-server don't need to be run directly.

Q: How do I run the mosh client and server separately?

If the mosh wrapper script isn't working for you, you can try running
the mosh-client and mosh-server programs separately to
form a connection. This can be a useful debugging technique.

1. Log in to the remote host, and run mosh-server.

It will give output like:

$ mosh-server
MOSH CONNECT 600044NeCCgvZFe2RnPgrcU1PQw
mosh-server (mosh 1.1.3)
Copyright 2012 Keith Winstein <mosh-devel@mit.edu>
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
[mosh-server detached, pid = 30261]

2. On the local host, run:

$ MOSH_KEY=key mosh-client remote-IPremote-PORT

where "key" is the 22-byte string printed by mosh-server (in this
example, "4NeCCgvZFe2RnPgrcU1PQw"), "remote-PORT" is the port number
given by the server (60004 in this case), and "remote-IP" is the IP address of the server. You can look up the
server's IP address with "host remotehost".

3. If all goes well, you should have a working Mosh connection. Information about where the process fails can help us debug why Mosh isn't working for you.

Q: With the mosh-server on FreeBSD or OS X, I sometimes get weird color problems. What's wrong?

This bug is fixed in Mosh 1.2. Thanks to Ed Schouten and Peter Jeremy for tracking this down.

Q: How do I contribute to mosh?

We welcome your contribution! Please join us in #mosh channel on Freenode IRC, visit us on GitHub,
or email mosh-devel@mit.edu. To contribute to our code base, please fork the repository on GitHub and open a pull request there.

Q: Who helped with mosh?

We're very grateful for assistance and support from:

Hari Balakrishnan, who advised this work and came up with the name.

Paul Williams, whose reverse-engineered vt500 state diagram is the basis for the Mosh parser.